The present invention relates to methods and apparatus for spray drying a liquid product into a dried powder without applying heat, or applying substantially low amounts of heat.
Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas (usually air). Spray drying technology has existed since the late 1800's and has continually evolved over the past century.
The spray drying process begins with a liquid solvent, commonly water, containing dissolved or suspended components such as an emulsion. The suspension includes a substance to be encapsulated (the load) and an amphipathic carrier (usually some sort of modified starch), which are homogenized as a suspension in the liquid solvent. The load is typically some constituent component(s) of a food, fragrance, medicament, etc., and the homogenized suspension is often referred to as a slurry.
Spray dryers use some type of atomizer, such as a spray nozzle, to disperse the slurry into a controlled spray having some relatively controlled droplet size. Depending on the process requirements, droplet sizes may range from about 10 to 500 microns in diameter. The most common applications require droplet sizes in the 50 to 200 micron range.
In conjunction with atomization, the slurry is fed into a drying chamber, usually a tower into which heated air is also introduced. The temperature of the air as it enters the drying chamber is well over the boiling point of water, usually in the range of 180-200° C. The heated air supplies energy for evaporation of volatile components of the liquid (the water) from the droplets. As the water evaporates, the carrier forms a hardened shell around the load, producing a dried powder.
Reference is made to FIG. 1, which illustrates a conventional spray drying system 50 and associated process. The process begins with making a slurry of ingredients. The ingredients include a liquid solvent, such as water 1, a carrier 2, and active ingredient(s) 3. In the typical process, the water 1 and carrier 2 are added into the solution tank 4 while stirring. The active ingredient 3 is then added to the tank 4 and stirred into the slurry. The active ingredient is either emulsified in the carrier fluid system or dissolved into it. In order for conventional spray drying processes to be commercially viable, typical slurry viscosities must be in the range of about 10-300 mPa-s.
The slurry formed in the solution tank 4 is delivered to an atomizer 6 using a feed pump 5 or other means of conveyance. The slurry enters the atomizer 6 and leaves the atomizer as a spray of liquid droplets 8, and the droplets 8 are introduced into a drying chamber 7. Concurrently, a feed of air is heated by a process heater 11 and supplied into the drying chamber 7 by a blower 10. The water evaporated from the droplets 8 enters the heated air as the atomized liquid droplets 8 dry to form solid particles after exposure to the incoming heated air.
The dried powder leaves the dryer chamber 7 along with the water vapor laden air, and is carried to a cyclone separator 12, which removes the dried particles from the circulating air stream and deposits the particles into a collection container 13. The water vapor laden air exits the collection container 13 and enters a baghouse 14, where very fine particles are removed before the water vapor laden air is sent into a condenser 9, via blower 15. The condenser 9 removes the water vapor from the process air, and the collected water may be re-used or discarded.
One of the prominent attributes of the traditional spray drying process is the high temperature of the inlet gas (typically on the order of 200° C.) leaving the heater 11 and entering the drying chamber 7, as well as the temperature of the outlet gas exiting the drying chamber 7, which is usually in excess of 100° C. Although the liquid droplets 8 are injected into the high temperature environment within the chamber 7, the droplets 8 do not actually reach the inlet gas temperature. The droplets 8, however, do become heated to a point at which considerable portions of desired constituents of the droplets (i.e., portions of the load) are undesirably modified, such as evaporated and/or oxidized. The undesirable modification to the load (load loss) leads to a reduction in flavor (in the case of food loads), a reduction in aroma (in the case of fragrances), etc. Essentially, evaporation and heat degradation of the load lowers the performance characteristics of the final powder product, and therefore results in a significant degradation of performance in commercial use and a significant loss of revenue.
The above disadvantageous characteristics of the conventional spray dry process have resulted in many process modifications and emulsion formulations to compensate for heat induced alterations in the load. This is especially true in the pharmaceutical industry, where excessive heating during spray drying leads to degradation of the active ingredient in a powdered medicament. This also presents a challenge to flavorists in the powdered flavor industry to design flavor formulations that can survive the drying process and deliver acceptable (although significantly flawed) flavor characteristics.
In view of the above, there are needs in the art for new methods and apparatus for carrying out the spray drying process, which reduce or eliminate the disadvantageous characteristics of the conventional spray dry process.